In general, an exhaust system for a diesel engine includes an exhaust gas aftertreatment device such as a Diesel Oxidation Catalyst (DOC), a Diesel Particulate matter Filter (DPF), a Selective Catalyst Reduction (SCR) device, and a Lean NOx Trap (LNT), for reducing carbon monoxide (CO), hydrocarbon (HC), particulate matter, nitrogen oxide (NOx), and the like.
An exhaust gas aftertreatment system employing a selective catalytic reduction (hereinafter referred to as ‘SCR’) device in exhaust gas aftertreatment systems for a diesel engine sprays a reduction agent such as a urea water solution into an exhaust pipe, thereby reducing nitrogen oxide in an exhaust gas into nitrogen and water. That is, when the reducing agent is sprayed into the exhaust pipe, the SCR device converts the reducing agent into ammonia (NH3) by the heat of the exhaust gas. Nitrogen oxide may be reduced to nitrogen gas (N2) and water (H2O) harmless to human and environment through a catalytic reaction of nitrogen oxide in the exhaust gas and ammonia with a SCR catalyst.
In this way, vehicles (such as trucks, buses, passenger cars, motorcycles, etc.) and ships, construction machines, and agricultural machinery with SCR devices are essentially equipped with urea water tanks.
The urea water tank needs a urea water solution supply system for spraying the urea water solution into the inside of the exhaust pipe and supplying it to the SCR device. However, the major component of the urea water is composed of urea and water, and the freezing point changes very much depending on the temperature. Since the freezing point is the lowest at −11.5° C. when the urea concentration is 32.5%, the urea water concentration is set at 32.5% for sale. Therefore, in the case of vehicles operating in cold conditions in winter, a heating device, and various sensors such as temperature and level may be attached to the urea water tank to prevent freezing of the urea water tank, and antifreeze or other additives may be added to prevent freezing.
In this case, since gas or the like is generated in the urea water tank, there is a possibility that a volume change occurs, so it is necessary to always maintain the pressure balance of the urea water tank.
As disclosed in Korean Patent Registration Publication No. 10-1205232 (on Nov. 27, 2012), a conventional urea water tank is provided with a tank housing in which urea water is stored, a sensor module which measures the water level, concentration and temperature of the urea water stored in the tank housing, and supplies the urea water to a urea water spraying device, and air vent portions which are installed on both sides of an upper portion of the tank housing and discharges the gas inside the tank housing to the outside.
However, since the air vent portion of the conventional urea water tank is in the form of a pipe and vertically installed on the upper side of the tank housing, pollutants such as external dust and foreign substances and moisture are introduced into the urea water tank through the air vent portion, to thereby cause a problem of polluting the urea water stored in the tank housing.